1. Field of the Invention
The present invention relates to stable graft polymer dispersions having low viscosity and containing less toxic by-products than found in commercially available graft polymer dispersions. More particularly, the invention relates to graft polymer dispersions prepared by the in situ polymerization of an ethylenically unsaturated monomer or mixture of monomers in a polyol in the presence of certain liquid free radical azo catalysts.
2. Prior Art
Graft polymer dispersions prepared by the in situ polymerization of an ethylenically unsaturated monomer or mixture of monomers in a polyol in the presence of a free radical catalyst and the use thereof in the preparation of polyurethanes are well known in the art as evidenced by U.S. Pat. Nos. 3,652,659; 3,875,258; 3,950,317, and U.S. Pat. Nos. Re. 28,715 and Re. 29,014. While these patents disclose various methods of preparing graft polymer dispersions, the most preferred method involves the simultaneous addition at a steady rate of a stream of monomer and a stream of catalyst dispersed in a portion of polyol to a reactor containing the remaining portion of the polyol. Numerous free radical catalysts are taught to be useful in the process described in the above patents with azobis(isobutyronitrile) generally described as the catalyst of preference. While azobis(isobutyronitrile) is a most suitable catalyst for the polymerization reaction, it is a solid at room temperature and only slightly soluble in the polyol and as a result thereof requires that it be continually stirred with the polyol so that it may be introduced into the reaction system in equally measured amounts. Moreover, when azobis(isobutyronitrile) decomposes during the reaction, one of its decomposition products, namely tetramethylsuccinonitrile, is an extremely toxic substance which is exceptionally difficult if not impossible to remove in toto from the graft polymer dispersion. The present invention relates to an improved process for the preparation of graft polymer dispersions comprising employing as the free radical catalyst certain liquid azo compounds.